The invention relates to an electric lamp comprising:
a lamp vessel which is closed in a vacuumtight manner and has a quartz glass wall; PA1 an electric element and a filling in the lamp vessel; PA1 metal foils embedded in the lamp vessel wall and each connected to a respective internal current conductor extending to the electric element and to a respective external current conductor made of molybdenum, PA1 which external current conductors issue from the wall to the exterior, each have a contact face at an end portion thereof and a centerline which substantially coincides with said contact face, the corresponding metal foil being welded to the said contact face.
Such an electric lamp is known from U.S. Pat. No. 4,002,939.
A body in the shape of a foil is used in lamps having a lamp vessel of quartz glass, i.e. glass having an SiO.sub.2 content of at least 96% by weight, for obtaining a vacuumtight seal on a metal body having a strongly different coefficient of linear thermal expansion, 54*10.sup.-7 and 45*10.sup.-7 for molybdenum and tungsten, respectively, as compared with approximately 10*10.sup.-7 for quartz glass. Depending on its width, the foil is thin, for example, approximately a hundred .mu.m for a width of 1 cm, to very thin, for example, a few tens of .mu.m, for example, 30 .mu.m for a width of, for example, 2 mm. The small thickness renders the foil mechanically vulnerable. The external and the internal current conductors are much thicker for providing a sufficient conductance and a sufficient mechanical strength.
The conductors are laterally connected to the metal foil with overlaps. When a metal foil with an external and an internal current conductor connected thereto is embedded in the wall of a lamp vessel, in a pinched seal or in a fused seal, the glass centers the current conductors in the said seal. The metal foil is also centered in a zone situated between the conductors. In the vicinity of the current conductors and laterally of these conductors, however, the foil cannot be centered. This is because the conductors themselves are centered, and the foil has its ends laterally of these centered conductors.
Because of its partly centered, partly eccentric situation, the metal foil follows a curved and/or kinked path. This path leads to tensional stresses in the foil which may lead to cracks. A reduced electric conductance, an increased heat generation in the wall, and a reduced luminous efficacy of the lamp are the results of this. There is even a risk of fracture of the foil, and thus of lamp failure immediately upon manufacture or shortly afterwards.
The external current conductor in the known lamp has a contact face which substantially coincides with its centerline. As a result, the foil has a substantially linear shape up to the welded joint at this conductor, and the creation of cracks and fractures is counteracted. The contact face is obtained in that the conductor is ground off at its end down to the centerline.
A disadvantage of this is the laboriousness thereof and the pollution caused by grinding. Another disadvantage is the reduced conductance of the external current conductor at the very spot of the welded joint owing to its smaller thickness.
DE 1 927 796 discloses an electric lamp in which the metal foil has a thickened portion on which a welded joint is made to a flat, widened end portion of the external current conductor. The metal foil follows a curved or kinked path at the external current conductor also in this lamp.